1. Field of the Invention
The present invention relates to a fabrication process and device of an optical wiring substrate.
2. Description of the Related Art
At an optical wiring substrate which is provided with optical wiring circuitry, which is structured as a circuit by optical waveguides (for example, an optical wiring circuit in which light is transmitted and the like by surface mounted-type light-emitting elements and light-receiving elements), light paths are changed by 90° between light paths at the surface mounted-type elements and light paths in the optical waveguides of the optical wiring substrate. Thus, the light is fed into the optical waveguides and emitted from the optical waveguides. End portions of the optical waveguides may accordingly be formed at an inclination of 45°, with reflection mirrors being provided at these inclined end faces.
Conventionally, in order to form an inclined surface at an end portion of an optical waveguide in such an optical wiring substrate, a photo mask is used in which density of a mask pattern at a region corresponding to the inclined face varies (increasing or decreasing) in a longitudinal direction of the optical waveguide. As a result, transmitted amounts of ultraviolet light exposed through a photoresist are varied, and an etching mask with an inclined structure is formed. The mask pattern is transferred by reactive ion etching or the like, and the optical waveguide end portion is worked to the desired inclined form (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 6-265738 (pages 3 to 4 and FIG. 1)). Alternatively, a thick film resist pattern may be formed to have an inclined side face by using a photo mask with a simple structure including a mask pattern for setting end vicinities of portions that are to become mirrors to be boundaries between transparency and shading. A core may be worked to be diagonal by using this resist pattern and dry-etching (see, for example, JP-A No. 2002-82242 (pages 3 to 4 and FIG. 2).
Furthermore, in recent years, it has become possible to create such a structure without a process of etching an optical wiring substrate (a large-area waveguide film or optical waveguides) by using a photo-bleaching method (see, for example, JP-A No. 6-265738 (pages 6 to 7 and FIGS. 5 and 6) and JP-A No. 2002-14241 (page 4)) or a direct exposure method. The photo-bleaching method uses a polymer material such as polysilane, BP-PMA or the like, whose refractive index is varied by light irradiation, in a core material. (BP-PMA is a polymethacrylate (PMA) polymer including a light sensitive base formed by a benzophenone residue (BP base).) the direct etching method uses a photo-curable resin. Accordingly, production can be made simpler.
However, in the example of JP-A No. 6-265738 mentioned above, because the photo mask has a particular structure, fabrication thereof is troublesome. Furthermore, in the technique of JP-A No. 2002-82242, two photolithography processes are required for working of mirror formation portions, and the production process is more complicated.
Moreover, in these conventional examples for carrying out photolithography processes, when an optical wiring substrate is to be fabricated with a number of fabrication levels, in order to provide a plurality of levels of optical wiring circuitry in a thickness direction (a lamination direction) of the substrate, the photolithography processing is carried out repeatedly. In such cases, with substrate materials such as polyimide and the like for which moisture absorption is high and dimensional variations are large, positional accuracy of the mirror formation portions are adversely affected by expansion and contraction of the material. Therefore, in order to achieve positioning with high accuracy, it is necessary to prepare a large number of photo masks with various dimensions. As a result, there are problems in that production processes are complicated and production costs rise.
Even with a photo-bleaching method, direct exposure method or the like not requiring an etching process, a photolithography process is necessary. Therefore, when a multi-layered circuit is to be formed as described above, high accuracy positioning and mirror formation are subject to the same issues.
Meanwhile, in recent years, various exposure apparatuses which employ spatial light modulation elements such as digital micromirror devices (DMD) and the like have been proposed for carrying out image exposure with light beams modulated in accordance with image data.
For example, a DMD is a mirror device in which numerous micromirrors, which alter angles of reflection surfaces thereof in accordance with control signals, are arranged in a two-dimensional pattern on a semiconductor substrate of silicon or the like. An exposure apparatus which utilizes such a DMD is structured with a light source which irradiates laser light, a lens system which collimates the laser light irradiated from the light source, a DMD which is disposed substantially at a focusing position of the lens system, and a lens system which focuses the laser light that has been reflected at the DMD onto a scanning surface. In this exposure apparatus, each micromirror of the DMD is switched “on” and “off” by control signals generated in accordance with image data or the like, and the micromirrors modulate the laser light. Thus, image exposure is carried out by modulated laser light. In this exposure apparatus, an exposure light amount (light intensity) can be controlled at each pixel, which is a single micromirror of the spatial light modulation element. Accordingly, this exposure apparatus has been considered as being favorable for maskless fabrication of the optical wiring substrate described above.